Normal-weight children get 16 more minutes of physical activity a day than their obese peers, a new study shows. And overall, girls do 20 minutes less physical activity a day than boys.

"This is a huge wake-up call to society," says Donna Spruijt-Metz, the senior author of the study and an associate professor of medicine at the University of Southern California.

A third of American children are overweight or obese. The government's physical activity guidelines recommend that kids and teens get an hour or more of moderate-intensity to vigorous aerobic physical activity a day.